ExxonMobil released details on what started a flash fire in the Beaumont refinery that injured 12 contract workers on Wednesday.

ExxonMobil spokeswoman Kathleen Jackson said the fire started when hydrocarbons released from a heat exchanger ignited. The heat exchanger was connected to a catalytic desulferization unit, or CHD, Jackson said in a prepared statement. The CHD unit, which was down for maintenance at the time of the fire, is used to remove sulfur from petroleum products, she said.

Jackson said the cause of the ignition and release of the hydrocarbons was not known as of Friday afternoon.

Twelve contract employees from three companies were injured in the fire.

UTMB spokeswoman Molly Danninmaeir said Friday afternoon that one of the victims was expected to be released Friday afternoon. Four of the injured were still listed in critical condition Friday afternoon and two were listed in serious condition, she said.